如何从Hibernate中的OneToMany Collection中仅选择一些属性？(How to Select only some properties from a OneToMany Collection in Hibernate?)

 TL; DR 
 
 如何获得B集合中只有少数属性的单个A对象？ 
 
 说明 
 
 我正在研究一个遗留项目，在这个项目中，人们有明智的想法将每个关系映射为EAGER。 
 
 现在，我们遇到了性能问题。 
 
 例如，我有A类，它有一个B对象列表。 该列表已在hibernate中映射为一个包，并且懒得加载（到目前为止，这么好）。 问题是，B急切地加载了整个怪异的字母表： 
 
 A.hbm.xml 
 
<hibernate-mapping>
    <class="A" table="a" lazy="false">
        // properties
        <bag name="listOfBs" inverse="true">
            <key column="a_id" [...]>
            <one-to-many class="B" />
        </bag>
</hibernate-mapping>
 
 B.hbm.xml 
 
<hibernate-mapping>
    <class="B" table="b" lazy="false">
        <many-to-one name="a" class="A" column="a_id" />
        // lots of other many-to-one mappings

        // properties

        // lots of one-to-many properties
</hibernate-mapping>
 
 所以我只需要B的4个属性，但它会获取每个相关对象！ 
 
 为了解决这个问题而不破坏一切，我试图使用hql查询只选择集合的几个列： 
 
"select a, b.field1, b.field2, b.field3, b.field4
 from A a inner join B b where a.id = :id"
 
 但我需要一个唯一的结果，因此以下查询会导致异常。 

TL;DR
 
How can I get a single A object with only a few properties in a B collection?
 
Explanation
 
I am working on a legacy project where people had the brillant idea to map every relationship as EAGER.
 
Now, we are having performance problems.
 
For instance, I have Class A, that has a list of B objects. That list has been mapped as a bag in hibernate, and loads lazily (so far, so good). The problem is, B eagerly loads the whole freaking alphabet:
 
A.hbm.xml
 
<hibernate-mapping>
    <class="A" table="a" lazy="false">
        // properties
        <bag name="listOfBs" inverse="true">
            <key column="a_id" [...]>
            <one-to-many class="B" />
        </bag>
</hibernate-mapping>
 
B.hbm.xml
 
<hibernate-mapping>
    <class="B" table="b" lazy="false">
        <many-to-one name="a" class="A" column="a_id" />
        // lots of other many-to-one mappings

        // properties

        // lots of one-to-many properties
</hibernate-mapping>
 
So I need only 4 properties of B, but it fetches every related object!
 
In order to solve this without breaking everything, I am trying to use a hql query to select only a few column of the collection:
 
"select a, b.field1, b.field2, b.field3, b.field4
 from A a inner join B b where a.id = :id"
 
But I need an Unique Result, so the following query causes an exception.

原文：https://stackoverflow.com/questions/18774101